A technology that forms the basis of current electronic information processing devices is the CMOS (Complementary Metal Oxide Semiconductor) technology, and continuous manufacturing process development and element miniaturization have been advanced to meet the demands of the times for high performance and large capacity. Such efforts have maintained Moore's law. In recent years, however, the miniaturization limit in the operation principle of CMOS transistors poses a real problem, and there are demands to reassess the roadmaps. Of these, the fields aimed at developing technologies based on new principles beyond the conventional CMOS technology are collectively called “beyond CMOS”. A beyond CMOS technology that has received particular attention is the field called “spintronics” that is intended to create new devices through the use of not only “electric charge” but also “spin” freedom of electrons.
A typical example of a device in spintronics is a magnetoresistive effect type solid-state magnetic memory (MRAM: Magnetic Random Access Memory) (for example, see Non-Patent Document 1). In the MRAM, laminate structures including magnetic elements are arranged in a matrix. Individual element recording information is read using the magnetoresistive effect of the magnetic element. Writing information is performed by controlling the magnetization direction of the magnetic element. Since the magnetization state is substituted as the recording state, the MRAM is nonvolatile in principle, and so is expected to serve as a nonvolatile memory that can satisfy all conditions including low power consumption, high speed, large capacity, write tolerance, compatibility with semiconductors, and so on. Especially, given that nonvolatility based on magnetism can ideally reduce the standby power required for information retention to zero, the device is attached importance as an innovative technology for realizing green technology.
Recently, in the above-mentioned laminate structure, ferromagnet magnetization can be dynamically controlled more directly by using the spin angular momentum transfer effect by spin polarized current injection (see Non-Patent Document 2). A high-frequency oscillation element (see Non-Patent Document 4), a high-frequency detection element (see Non-Patent Document 5), and the like that actively use the giant magnetoresistive effect in a tunnel magnetoresistive element (Non-Patent Document 3), and not only the magnetization reversal control in the MRAM and the like, have been proposed and experimentally verified.
Meanwhile, “magnonics” has also received attention (see Patent Documents 1 and 2 and Non-Patent Document 6). Magnonics is intended to create new devices through the use of “spin wave” that propagates as wave motion with each spin precession having a different phase while mediating exchange interaction or dipolar interaction in a continuous ferromagnet.
Furthermore, research has also begun on signal transmission and magnetic function elements using “pure spin current” which is the flow of spin angular momentum without a charge current (Patent Documents 3 and 4). The pure spin current does not involve Joule heat generation, and so is expected to enable low-loss information transmission.
The mounting expectations for devices using the spin freedom of electrons mentioned above have made it important to establish a ferromagnetic resonance dynamics control method for controlling the spin state with high efficiency. Ferromagnetic resonance is excited by, under the application of a static magnetostatic field, applying a high-frequency magnetic field to a ferromagnet in the direction orthogonal to the magnetostatic field. In the case where the frequency of the applied high-frequency magnetic field matches the natural resonance frequency of the ferromagnet, the applied energy is absorbed with high efficiency and resonance dynamics are excited.
Of these magnetic devices, especially the spin wave or the pure spin current can be generated by such FMR dynamics excitation, and so its control method is critical in applied technologies.
In the magnetic recording field, too, the magnetization reversal magnetic field reduction using resonance motion has been attempted recently, and has attracted attention as a next-generation magnetic recording method (Non-Patent Document 7).
As the high-frequency magnetic field necessary for ferromagnetic resonance excitation, a current magnetic field generated by the application of an electromagnetic field using a cavity or the application of a high-frequency current to a transmission path such as a coplanar waveguide formed on a substrate has been used. However, since the magnetic field generated by such a method has a spatial distribution, for example in the case where the element scale is of the order of several hundred nm or less, an interference between adjacent elements or the like poses a problem. The use of the above-mentioned resonance excitation method employing the spin angular momentum transfer effect (see Non-Patent Document 5) enables local resonance excitation restricted within an element. However, the method using the current as the drive power, such as the current magnetic field or the spin angular momentum transfer effect, has a problem of high power consumption.
Consider power necessary for magnetization reversal control, as an example. When expressed with room-temperature energy kBT (KB is the Boltzmann constant and T is the temperature) as an index, energy necessary for magnetization reversal of a magnetic thin-film element of the order of 100 nm per side is of the order of 106 to 107 kBT in the case of the current magnetic field, and of the order of 105 to 106 kBT in the case of the spin angular momentum transfer effect. On the other hand, energy necessary for information retention for 10 years or more is about 60 kBT. This demonstrates the low efficiency of the current-based spin control method.
One main breakthrough for solving such a problem is to establish a spin state control method using an electric field. Magnetization state control and in particular magnetization reversal control using an electric field have been attempted in various forms. Typical examples include: a method of connecting a magnetic thin film to a piezo element and controlling the magnetostrictive effect of the magnetic thin film with the use of the distortion by voltage application to the piezo element (see Patent Document 5); a method of controlling magnetism by carrier concentration control of a ferromagnetic semiconductor (see Non-Patent Document 8); a method using the electromagnetic effect in a single-phase multiferroic material (see Patent Document 6) and a ferromagnetic/multiferroic composite structure (Non-Patent Document 9), and so on.
However, these methods fail to establish a technology that satisfies all requirements considered necessary in terms of application, namely:
(1) capable of room-temperature operation;
(2) high repeated operation tolerance; and
(3) simple production process.
As a technology that can satisfy all of these three conditions, there is a method of directly controlling, by voltage application, the magnetic anisotropy of an ultrathin metal magnetic layer of the order of several atomic layers, and a magnetic easy axis control method and a magnetization reversal control method using this phenomenon have been proposed (Patent Document 7). A significant feature of this technology is that it has the same basic structure as a ferromagnetic tunnel junction element which is a spintronics device important in terms of application, and can be relatively easily introduced to the existing process.
These electric field control methods are, however, all limited to the electrostatic field application effect, and no ferromagnetic resonance dynamics excitation control method by high-frequency electric field application has been proposed.
If electric field-driven type ferromagnetic resonance excitation by high-frequency voltage application becomes possible, it serves as an important technology for fundamentally replacing the signal input structure of a device that uses electron spin resonance, and allows various low-power magnetic function elements to be provided.